The human ear doesn't hear one frequency at a time.
A way to physically see what the ear is experiencing is to take a pool of water. Now make some waves. Splash around. See how the entire tub gets full of up and down motion all over the place?
In the air, similar things happen; but instead of "up and down" those waves correspond to changes in pressure. (Air is much more compressible than water; water will expand upwards a whole bunch under a pressure wave, while air will mostly just compress).
Now stick a small toy in the water as you splash about. See how it floats up and down?
Build two mesh tubes. In those mesh tubes, place a float. Have them free to float up and down with the waves. Now splash around. The movement of those floats in the tubes is basically what it is our ears are detecting.
With that simple signal, our ears and brain can determine that there are 3 waves in the bathtub, and the direction and size and characteristics of each, even though the bathtub is sloshing around like crazy.
To do this, first of all make the "mesh tube" directional: it is now shaped like an rectangle, and only short one side is open to letting waves in. Add a funnel at the entrance to guide waves in.
Next, instead of one float, have a whole bunch of tiny floats along the oval-shaped ear canal. Now waves go in, go down the rectangle, bounce off the end, then come back. Each of the floats independently picks up upwards and downward motion of the waves.
Hook those floats up to a nervous system, which filters and combines the signals from both rectangular tubes. Then attach that to a brain, which builds a 3d model of where the waves are coming from and what kind of motion each kind of wave has.
Now, a signal of a fixed "pure" frequency in the bath tub causes a certain kind of signal to be picked up by the ears. But more complex signals are equally handled by this system.
Frequency breakdown is something we do mathematically, partly because it models some of the stuff our ears do. Signals of a specific frequency (rate of change in the pressure wave) excite certain of the "floats" (actually hairs) in our ear canal, and the math we do to turn a set of pressure changes (sound) into pure frequencies is [b]similar to[/b] part of the system of how our ear canal hairs convert the pressure changes into signals.
Our ears are evolved to pick up such frequencies, because pure frequencies correspond to repeated vibration of some object. And repeated vibration is something a lot of evolutionary interesting things do; when something partly supported gets an impact, it vibrates for a bit. That snap of a twig, clatter of a rock, rumble of lungs, rustle of a leaf, or thump of a footstep is very useful to detect other creatures who we might want to eat, or might want to eat us.
So while we hear pure frequencies, we are in effect amplifying their importance out of the jumble of pressure changes our ears are detecting. The other pressure changes still happen, are still detected, and still go into our brain.